The present invention relates generally to photovoltaic cells, and more particularly to an improved solar cell which may be used to more efficiently transform solar energy into electrical energy.
A variety of photovoltaic cells, solar cells, and related devices have been proposed. For example, one conventional solar cell structure, used to convert solar photon radiation into electric current, comprises a layered extrinsic semiconductor. The conventional solar cell operates by converting incident photon radiation into electron-hole pairs within the semiconductor. These electron-hole pairs are then collected and separated at a P-N junction within the semiconductor to provide the electrical current produced by the solar cell.
In a conventional solar cell, the semiconductor has opposing incident and collection sides, with the incident side of the cell receiving the photon radiation. The semiconductor has a strongly doped (i.e., designated impurities have been added to a pure semiconductor material) negative conduction type or N-type layer (hereinafter designated an N.sup.+) lying adjacent the incident side. The N.sup.+ region serves as a cathode for the solar cell. An anti-reflection (AR) coating is applied over the N.sup.+ region. A strongly doped positive or P-type layer (hereinafter designated as P.sup.+) lies adjacent the collection side of the cell. The P.sup.+ region serves as an anode for the solar cell. The semiconductor also has a lightly doped base layer which is slightly positive conduction type or P-type (hereinafter designated as P.sup.(-)), sandwiched between the P.sup.+ and N.sup.+ layers. Due to the longer diffusion length for electrons in silicon, the most commonly used silicon solar cell has this complementary N.sup.+ P.sup.(-) P.sup.+ cell structure. This layering of oppositely doped layers forms a P-N junction within the solar cell.
When the conventional solar cell is exposed to a light source, such as the sun, the impinging photons create electron-hole pairs in the semiconductor, with the electron portion comprising a negative charge carrier and the hole portion comprising a positive charge carrier. The impinging photons also cause a depletion region to be formed across the P-N junction, with the number of positive and negative charge carriers within the depletion region being substantially equal. All of the carriers created in the depletion region contribute to the photo-current output.
For carriers created in the N.sup.+ region near the incident side to contribute to the photo-current output, they must diffuse through the thickness of the N.sup.+ layer to reach the P-N junction. However, due to surface recombination, some of the carriers recombine with atoms in the lattice of the N.sup.+ region prior to reaching the P-N junction. Thus, these recombined carriers do not contribute to the solar cell current, resulting in a loss of solar cell efficiency.
For carriers created deep in the bulk of the P.sup.(-) base region to contribute to the photo-current, these carriers must diffuse and travel to the edge of the depletion region. However, the width of the depletion region extending into the P.sup.(-) base layer is relatively small, basically due to the small bias across the P-N junction. During this travel, the carriers recombine with atoms in the lattice of the P.sup.(-) base layer, which results in a further loss of efficiency. Furthermore, the loss of carriers in the P.sup.(-) region can be severe if the carrier lifetime, that is, the length of time before recombination occurs, is reduced.
Thus, the collection of the photo-current forming electron-hole pairs at the P-N junction takes place in three ways: (1) by the generation of electron-hole pairs in the depletion region surrounding the P-N junction; (2) by the diffusion of minority carriers in the heavily doped N.sup.+ layer adjacent the P-N junction; and (3) by the diffusion of minority carriers in the lightly doped P.sup.(-) base layer.
In the conventional solar cell, all of the electron-hole pairs generated in the depletion region (item 1 above) are collected by the P-N junction, and thus contribute to the photo-current generated by the cell. However, only a fraction of the hole-electron pairs generated in the lightly doped P.sup.(-) base layer (item 3 above) are able to actually diffuse to the P-N junction. This recombination of carriers in the P.sup.(-) base region causes a loss of spectral response at lower photon energies, resulting in an overall decrease in the output voltage of the solar cell.
Additionally, the conventional solar cell suffers a reduced spectral response at higher photon energies. This results from a loss of carriers generated in the N.sup.+ region (item 2 above) when the incident photons have high energies. The carriers are lost due to recombination of some of the generated carriers within the N.sup.+ region. Furthermore, for silicon solar cells, there is zero spectral response at photon energies of less than 1.1 eV because no carriers can be generated across the band gap of the cell when the photon energy falls below this value.
Thus, the conventional layered extrinsic semiconductor solar cell structure suffers a variety of disadvantages. Therefore, a need exists for an improved solar cell for converting solar or photon energy into electrical energy, such as electric current, which is directed toward overcoming, and not being susceptible to, the above limitations and disadvantages.